Domain inversion is a phenomenon in which the polarization of a ferroelectric material is inverted forcibly. With the use of this phenomenon, domain-inverted regions are arranged periodically inside the ferroelectric material to form a domain-inverted structure. The domain-inverted structure is used, e.g., for an optical frequency modulator that utilizes surface acoustic waves, an optical wavelength conversion element that utilizes the domain inversion of nonlinear polarization, or an optical polarizer that utilizes a domain-inverted structure in the form of a prism or lens. In particular, an optical wavelength conversion element with very high conversion efficiency can be produced by periodically inverting the nonlinear polarization of a nonlinear optical substance. When this optical wavelength conversion element is used to convert the wavelength of light of a semiconductor laser or the like, a small short-wavelength light source can be provided and applied to the field of printing, optical information processing, or optical measurement and control.
The ferroelectric material has a displacement of charge of the crystals due to spontaneous polarization. The direction of the spontaneous polarization can be changed by applying an electric field opposite to the spontaneous polarization. The direction of the spontaneous polarization varies depending on the type of crystal (material). The crystals of a substrate made of LiTaO3, LiNbO3, or a mixed crystal of them, i.e., LiTa(1−x)NbxO3 (0≦x≦1), have the spontaneous polarization only in the C-axis direction. Therefore, the polarization of these crystals is present in either of two directions (+ direction and − direction) along the C axis. The application of an electric field rotates the polarization by 180 degrees opposite to its original direction. This phenomenon is called domain inversion. The electric field required for the domain inversion is called a polarization electric field. The polarization electric field is about 20 kV/mm at room temperature for LiNbO3 or LiTaO3 crystals, and about 5 kV/mm for MgO:LiNbO3.
When the ferroelectric material is transformed into crystals having a single polarization direction, the process is referred to as “single domain of polarization”. In general, the polarization is changed to a single domain by applying an electric field at high temperatures after crystal growth.
As a conventional method for forming periodically domain-inverted regions, e.g., JP 4(1992)-19719 discloses that a comb-shaped electrode is formed on a LiNbO3(Lithium niobate) substrate and a pulse electric field is applied to the comb-shaped electrode. In this method, the comb-shaped electrode is formed on the + C plane of the LiNbO3 substrate, while a planar electrode is formed on the − C plane. The + C plane is grounded, and a pulse voltage with a pulse width of 100 μs is applied to the planar electrode on the − C plane so that the polarization is inverted by the pulse electric field applied to the substrate. The electric field required to invert the polarization is not less than about 20 kV/mm. When the electric field of such a value is applied to a thick substrate, the crystals of the substrate may be damaged. However, a substrate having a thickness of about 200 μm can avoid the crystal damage caused by the applied electric field and also can form domain-inverted regions at room temperature. Thus, the domain-inverted regions can be deep enough to penetrate the substrate.
A domain-inverted structure with a short period of 3 to 4 μm is necessary to achieve a high-efficiency optical wavelength conversion element. When an electric field is applied so as to form domain-inverted regions, the polarization directly under the electrode is inverted, and then the domain-inverted regions expand in the lateral direction of the substrate. Therefore, it is difficult to provide a short-period domain-inverted structure. To solve this problem, a conventional method employs a pulse width of about 100 μs and applies a pulse voltage to the electrode for a short time, thereby providing a short-period domain-inverted structure.
As a method for forming a short-period domain-inverted structure in a Mg-doped LiNbO3 substrate (referred to as MgLN in the following), e.g., JP 6(1994)-242478 discloses a method for forming a periodically domain-inverted structure in a MgLN of a Z plate. In this method, a comb-shaped electrode is formed on the + Z plane of the MgLN, and the substrate is irradiated with corona from the underside, thus providing a domain-inverted structure that has a period of 4 μm and penetrates the 0.5 mm thick substrate.
JP 9(1997)-218431 discloses a method for forming a domain-inverted structure in an off-cut MgLN. An electrode is formed on the off-cut MgLN whose polarization direction slightly tilts from the substrate surface, and a voltage is applied to the electrode, thus providing an acicular domain-inverted structure. The domain-inverted regions grow in the polarization direction of the crystals, and the domain-inverted structure has a period of about 5 μm.
However, it has been difficult to form a fine domain-inverted structure in a Mg-doped LiTa(1−x)NbxO3 (0≦x≦1) substrate of a Z plate. Although the domain-inverted structure can be formed in the off-cut substrate by applying an electric field with the conventional method, only a complicated technique such as corona poling is known for forming a fine uniform domain-inverted structure in the Z-plate substrate. In the corona poling, charged particles are deposited on the substrate to generate an electric field for inverting the polarization. However, there is a limit to the magnitude of the electric field generated by the charged particles. Therefore, the thickness of a substrate available to form a domain-inverted structure is limited to about 0.5 mm, and the domain-inverted structure cannot be formed in the substrate having a large thickness of more than 1 mm. On the other hand, the application of a voltage using the electrode is effective in forming a domain-inverted structure in the off-cut substrate. However, such a system is not useful to form a domain-inverted structure widely and uniformly in the Z plate.
JP 2001-66652 discloses that a comb-shaped electrode is formed on a MgLN of a Z plate, and a voltage is applied to the comb-shaped electrode, thereby providing a periodically domain-inverted structure. This method has the advantage of forming the periodically domain-inverted structure uniformly. However, the domain inversion is limited to part of the end of the electrode. Thus, it is difficult to form a domain-inverted structure deeply and uniformly in a wide range of the substrate under the electrode.